To construct a wheel-driving unit of the above type and support the wheels on the suspension so as to be rotatable with respect to the suspension, there are used various bearing units in which outer and inner rings are rotatably combined together through rolling elements. Also, in the case of a wheel-driving bearing unit which is used not only to support drive wheels on the independent suspension but also to drive and rotate the drive wheels, it is necessary that it is combined with a constant velocity joint and, through this combination, it transmits the rotary motion of the drive shaft to these drive wheels smoothly (that is, while securing the constant velocity property of the rotary motion) regardless of not only the mutual shift between the differential gears and drive wheels but also the steering angles that are applied to the drive wheels. Now, FIG. 39 shows an ordinary wheel-driving bearing unit structured for the above purpose such that a wheel-supporting bearing unit 1 and a wheel-side constant velocity joint 2 are combined together.
The wheel-supporting bearing unit 1 is structured such that a hub 4 and an inner ring 5 are rotatably supported on the inside diameter side of an outer ring 3 through a plurality of rolling elements 6, 6. The outer ring 3 is connected and fixed to a knuckle 8 (see FIG. 40 which will be discussed later), which constitutes the suspension, through an outwardly-facing flange-shaped mounting portion 7 formed in the outer peripheral surface of the outer ring 3, while the outer ring 3 does not rotate even when it is in use. By the way, conventionally, there is also known a structure in which the outer peripheral surface of the outer ring 3 is formed as a simple cylindrical surface (which does not include the mounting portion 7) and is fitted with and fixed to the inner surface of a support hole formed in the knuckle 8. Also, in the inner peripheral surface of the outer ring 3, there are formed a double row of outer raceways 9, 9; and, the hub 4 and inner ring 5 are supported on the inside diameter side of the outer ring 3 in such a manner that they are concentric with the outer ring 3 and can be rotated.
In the case of the hub 4, on the portion thereof that is situated near to the outer (with respect to the axial direction thereof, when the wheel-supporting bearing unit 1 is incorporated into a car, the width-direction outer side of the car: and, this applies similarly in the whole of the present specification; that is, the left side of the respective figures except for FIGS. 4, 7, 10, 13, 25, 26, 28, 32, 37 and 38) end of the outer peripheral surface of the hub 4, there is disposed a flange 10 which is used to support wheels (drive wheels). Also, in the middle portion of the outer peripheral surface of the hub 4, there is formed a first inner raceway 11 and, similarly, the inner ring 5 having a second inner raceway 13 formed in the outer peripheral surface thereof is fitted with and fixed to the outer surface of a small-diameter stepped portion 12 formed in the portion that is near to the inner (with respect to the axial direction thereof, when the wheel-supporting bearing unit 1 is incorporated into a car, the width-direction central side of the car: and, this applies similarly in the whole of the present specification; that is, the right side of the respective figures except for FIGS. 4, 7, 10, 13, 25, 26, 28, 32, 37 and 38) end portion of the outer peripheral surface of the hub 4. And, in the central portion of the hub 4, there is formed a spline hole 14.
On the other hand, the wheel-side constant velocity joint 2 comprises an outer ring 15 for a constant velocity joint, an inner ring 16 for a constant velocity joint, and a spline shaft 17. The outer ring 15 for a constant velocity joint and spline shaft 17 form a drive member 18. That is, the spline shaft 17 is disposed on the outer end portion of the drive member 18 and can be engaged with the spline hole 14; and, the outer ring 15 for a constant velocity joint is disposed on the inner end portion of the drive member 18. In a plurality of portions in the circumferential direction of the inner peripheral surface of the outer ring 15 for a constant velocity joint, there are formed outside engaging grooves 19, 19 in such a manner that they respectively extend at right angles to the present circumferential direction. Also, in the case of the inner ring 16 for a constant velocity joint which forms the wheel-side constant velocity joint 2, in the central portion thereof, there is formed a second spline hole 20 and, in the portions of the outer peripheral surface thereof that correspond to the outside engaging grooves 19, 19, there are formed inside engaging grooves 21, 21 in such a manner that they respectively extend at right angles to the circumferential direction of the outer peripheral surface of the inner ring 16. Balls 22, 22 are interposed between the inside engaging grooves 21, 21 and outside engaging grooves 19, 19 and are held by a retainer 23 in such a manner that they are allowed to roll along these engaging grooves 21, 19. By the way, the shapes of the composing parts of the wheel-side constant velocity joint 2 are similar to those of a constant velocity joint of a Rzeppa type or a Birfield type and are not related to the gist of the invention and, therefore, the detailed description thereof is omitted here.
In the case of the above-mentioned wheel-side constant velocity joint 2 and the previously-mentioned wheel-supporting bearing unit 1, the spline shaft 17 is inserted into the spline hole 14 of the hub 4 from the inside to the outside with respect to the axial direction of the hub 4. And, a nut 25 is threadedly engaged with a male screw portion 24 formed in such portion of the outer end portion of the spline shaft 17 that projects from the outer end face of the hub 4 and, by fastening the nut 25 and male screw portion 24 further, they can be connected and fixed to each other. In this state, the inner end face of the inner ring 5 is contacted with the outer end face of the outer ring 15 for a constant velocity joint, thereby preventing the inner ring 5 from shifting in a direction where it can slip out of the small-diameter stepped portion 12. At the same time, a proper preload is applied to the respective rolling elements 6, 6.
Further, in a state where the present wheel-driving unit is assembled into the suspension of the car, a male spline portion 27 formed in the outer end portion of a drive shaft 26 is spline engaged into the second spline hole 20 formed in the central portion of the inner ring 16 for a constant velocity joint. And, a retaining ring 29, which is secured to a securing groove 28 formed so as to exist over the entire periphery of the outer peripheral surface of the outer end portion of the male spline portion 27, is engaged with a securing stepped portion 30 formed in the peripheral edge portion of the outer end of the second spline hole 20, thereby preventing the male spline portion 27 from slipping out of the second spline hole 20.
By the way, the inner end portion of the drive shaft 26 is connected to the output portion of a tripod-type constant velocity joint which is a differential-side constant velocity joint. And, the tripod-type constant velocity joint, the drive shaft 26 and the wheel-side constant velocity joint 2 cooperate together in forming the above-mentioned constant velocity joint unit. In a state before the present wheel-driving unit is assembled into the suspension of the car, the present constant velocity joint unit is combined with the wheel-supporting bearing unit 1 to thereby form the above-mentioned wheel-driving unit.
Also, in the specification of U.S. Pat. No. 4,881,842, there is disclosed such a wheel-driving bearing unit as shown in FIG. 40. In the second example of the conventional structure shown in FIG. 40 as well, a hub 4a is rotatably supported inside an outer ring 3 fixed to a knuckle 8 by a double row of rolling elements 6, 6. And, the spline shaft 17 of a drive member 18a is spline engaged into a spline hole 14 formed in the central portion of the hub 4a. In the outer end face of the spline shaft 17, there is formed a securing portion 31 for securing thereto a tool which is used to draw the spline shaft 17 into the spline hole 14. An incomplete-circular-ring-shaped retaining ring 33 is bridgingly interposed between a securing groove 32, which is formed in the near-to-leading-end portion of the outer peripheral surface of the spline shaft 17 and serves as an inside engaging portion, and a securing stepped portion 35 which is formed in such portion of the inner peripheral surface of the hub 4a that is opposed to the securing groove 32 and also which serves as an outside engaging portion, thereby preventing the spline shaft 17 from slipping out of the hub 4a. Also, in this state, an elastic ring 34 is elastically compressed between the hub 4a and an outer ring 15 for a constant velocity joint disposed on the drive member 18a. 
Also, an inner ring 5 having a second inner raceway 13 formed in the outer peripheral surface thereof is fitted with the outer surface of the near-to-inner-end portion of the hub 4a and, at the same time, a retaining ring 37 is secured to a securing groove 36 formed over the entire periphery of the outer peripheral surface of such portion of the inner end portion of the hub 4a that projects inwardly in the axial direction beyond the inner end face of the inner ring 5. And, the retaining ring 37 holds the inner end face of the inner ring 5 to thereby prevent the inner ring 5 from shifting in the axial direction thereof.
According to the second example of the conventional structure disclosed in the above-cited U.S. Pat. No. 4,881,842, it is easy to connect together a wheel-supporting bearing unit 1a and a wheel-side constant velocity joint 2a. That is, in the case of the first example of the conventional structure shown in FIG. 39, to connect together the wheel-supporting bearing unit 1 and wheel-side constant velocity joint 2, there is necessary the troublesome operation in which the nut 25 is threadedly engaged with the male screw portion 24 formed in the leading end portion of the spline shaft 17 and they are then fastened further. In case where the operation to connect together the wheel-supporting bearing unit 1 and wheel-side constant velocity joint 2 is troublesome, the operation gives rise to an increase in the cost necessary for assembling of the wheel-driving bearing unit. Also, separate provision of the male screw portion 24 and nut 25 increases both size and weight of the wheel-driving bearing unit. On the other hand, in the case of the second example of the conventional structure, to connect together the wheel-supporting bearing unit 1a and wheel-side constant velocity joint 2a, the retaining ring 33 may only be interposed between the inner peripheral surface of the outer end portion of the hub 4a and the outer peripheral surface of the outer end portion of the spline shaft 17. For this reason, in the case of the second example of the conventional structure, the operation to connect together the wheel-supporting bearing unit 1a and wheel-side constant velocity joint 2a can be executed easily, which only can reduce the cost necessary for assembly of the structure but also can reduce the size and weight thereof.
Further, in JP-A-10-264605, there is disclosed such a wheel-driving bearing unit as shown in FIG. 41. In the case of a wheel-supporting rolling bearing unit 1b forming part of the present wheel-driving bearing unit, to the inner end portion of a hollow hub 4b, there is connected through an intermediate seat 38 an outer ring 15a for a constant velocity joint which corresponds to a drive member set forth in the appended claims thereof and forms a wheel-side constant velocity joint 2b. Of the two inner and outer peripheral surfaces of the intermediate seat 38 which is formed in a short-cylindrical shape, in the inner peripheral surface thereof, there is formed an inside-diameter-side female spline portion 39; and, in the outer peripheral surface thereof, there is formed an outside-diameter-side male spline portion 40 which corresponds to a first spline portion set forth in the appended claims of the present specification. The intermediate seat 38 is assembled to the inner end portion of the hub 4b by spline engaging an inside-diameter-side male spline portion 41 formed in the outer peripheral surface of the inner end portion of the hub 4b with the inside-diameter-side female spline portion 39 with no shaky motion between them. And, the inner end face of the intermediate seat 38 is held by a caulk portion 42 formed in the inner end portion of the hub 4b, thereby fixing the intermediate seat 38 to the inner end portion of the hub 4b with no shaky motion between them. By the way, in the above-cited JP-A-10-264605, there is also disclosed a structure in which the inner ring and intermediate seat are formed as an integral body. This structure is also the subject of the present invention.
On the other hand, with the outside-diameter-side male spline portion 40 formed in the outer peripheral surface of the intermediate seat 38, there is spline engaged an outside-diameter-side female spline portion 43 which is formed in the inner peripheral surface of the outer end portion of the outer ring 15a for a constant velocity joint and corresponds to a second spline portion set forth in the appended claims of the present specification. And, a retaining ring 33a is bridgingly interposed between the outside-diameter-side female spline portion 43 and outside-diameter-side male spline portion 40 that are mutually spline engaged, thereby preventing the outer ring 15a for a constant velocity joint and the intermediate seat 38 from being separated from each other. That is, the incomplete-circular-ring-shaped retaining ring 33a is bridgingly interposed between an inside securing groove 44, which is formed over the entire periphery of the outer peripheral surface of the intermediate seat 38 and corresponds to a first engaging portion set forth in the appended claims of the present specification, and an outside securing groove 45 formed over the entire periphery of the inner peripheral surface of the outer end portion of the outer ring 15a for a constant velocity joint and corresponding to a second engaging portion set forth in the appended claims of the present specification, thereby preventing the outer ring 15a for a constant velocity joint and the intermediate seat 38 from moving in the axial direction of the present wheel-driving bearing unit.
While the structure of the connecting portion between the hub 4b and the outer ring 15a for a constant velocity joint is as described above, the hub 4b is rotatably supported on the inside diameter side of the outer ring 3 by a double-row angular-type ball bearing. An inner ring 5 forming this ball bearing is held and fixed by and between the outer end face of the intermediate seat 38 and a stepped surface 46 existing in the outer end portion of a small-diameter stepped portion 12 formed in the outer peripheral surface of the middle portion of the hub 4b. 
In the case of the above-structured wheel-driving bearing unit disclosed in the above-cited JP-A-10-264605, since the spline shaft 17 can be omitted from the second example of the conventional structure shown in FIG. 40, the cost and weight thereof can be reduced further by an amount corresponding to the omission of the spline shaft 17.
In all of the above-mentioned conventional structures, due to a slight clearance existing in the spline engaged portion for transmission of the rotary power, there is a possibility that there can be generated shaky motion in the circumferential direction thereof. Firstly, in the case of the first and second conventional structures shown respectively shown in FIGS. 39 and 40, there is a possibility that, in a neutral state (in a state where torque is not transmitted between the spline shaft 17 and hub 4, 4a), there can be generated a slight clearance between the side surfaces (which are opposed to each other in the circumferential direction) of the respective teeth of the spline engaged portion 49 composed of a male spline portion 47, which is formed in the outer peripheral surface of the spline shaft 17 and corresponds to a second spline portion set forth in the appended claims of the present specification, and a female spline portion 48 which is formed in the spline hole 14 formed in the central portion of the hub 4 and corresponds to a first spline portion as set forth in the appended claims of the present specification.
Between the spline shaft 17 and hub 4, 4a, there is transmitted torque through the spline engaged portion 49. However, as described above, in case where there is generated a slight clearance between the side surfaces of the respective teeth of the male and female spline portions 47, 48, in the speed increasing or reducing time of the car, the male and female spline portions 47, 48 are shifted with respect to each other in the circumferential direction, although slightly. And, because of this relative shift, the male and female spline portions 47, 48 can be collided with each other, which raises a possibility that there can be generated teeth striking sounds which are offensive to the ear of an occupant of the car or a pedestrian present in the periphery of the car. Such problem can be found also in the case of the third conventional structure shown in FIG. 41, that is, in the spline engaged portion 49a composed of the outside-diameter-side male spline portion 40 formed in the outer peripheral surface of the intermediate seat 38 and the outside-diameter-side female spline portion 43 formed in the inner peripheral surface of the outer end portion of the outer ring 15a for a constant velocity joint.
In view of the above, there has been made a proposal that, when enforcing the first conventional structure shown in FIG. 39, for example, each of the teeth of the male spline portion 47 may be inclined (for example, about 10 min.) with respect to the center axis of the spline shaft 17, that is, each tooth may be shaped so as to have such torsion angle. Also, in this case, the respective teeth of the female spline portion 48, which is to be spline engaged with the male spline portion 47, are formed so as to be parallel to the center axis of the spline shaft 17. By the way, the surface of the male spline portion 47 is hardened in the following manner: that is, the outer peripheral surface of the spline shaft 17 is rolled to thereby form a given shape having the above-mentioned torsion angle and, after then, it is high frequency quenched. On the other hand, in the case of the female spline portion 48, the inner peripheral surface of the spline hole 14 is broached to thereby form a given shape and, after then, it is left un-quenched.
In a state where, while the male and female spline portions 47, 48, which have been formed into the above-mentioned respective shapes, are spline engaged with each other, the wheel-supporting bearing until 1 and wheel-side constant velocity joint 2 are connected to each other, the side surfaces of the respective teeth of the male and female spline portions 47, 48 are pressed against each other in the axial-direction two end portions thereof, thereby being able to prevent generation of the above-mentioned teeth striking sounds in the running operation of the car. On the other hand, there can also be expected a structure in which the respective teeth of the male and female spline portions 47, 48 are formed parallel to the axial direction and the clearances between the teeth mutually adjoining in the circumferential direction are regulated to thereby cause the side surfaces of the respective teeth of the male and female spline portions 47, 48 to be strongly pressed against each other over the entire length thereof in the axial direction. In the case of this structure as well, generation of the teeth striking sounds can be prevented. However, in this structure, the shapes and dimension accuracy must be regulated very strictly, which unfavorably not only increases the cost of the structure but also requires an excessively large force (pressure insertion force) for pressure insertion of the spline shaft 17 into the spline hole 14. In this respect, in the case of the previously-described structure in which the male spline portion 47 is formed into a shape having a torsion angle to thereby prevent occurrence of the teeth striking sounds, since the side surfaces of the respective teeth of the male and female spline portions 47, 48 are pressed against each other only in the axial-direction two end portions thereof, the pressure insertion force can be reduced.
However, in case where, in order to prevent occurrence of the teeth striking sounds, the male spline portion 47 is formed into a shape having a torsion angle, the pressure insertion force can vary greatly due to the delicate variations of the torsion angle. For this reason, in case where the male spline portion 47 varies in shape, there is also a possibility that the pressure insertion force can increase up to an excessive value. Therefore, the shapes and dimension accuracy must be secured to a certain degree, which makes it difficult to reduce the cost of the structure sufficiently. By the way, according to the first conventional structure shown in FIG. 39, even in case where the pressure insertion force becomes excessively large in the above manner, by tightening the nut 25 with respect to the male screw portion 24 strongly, the nut 25 is able to generate an axial-direction force (axial force), thereby being able to obtain the pressure insertion force that is necessary. However, as in the second conventional structure shown in FIG. 40, in case where, in order to facilitate the assembling of the wheel-driving bearing unit, the connection between the wheel-supporting bearing unit 1a and wheel-side constant velocity joint 2a is carried out by the retaining ring 33, the retaining ring 33 is not allowed to generate the above axial force. In the case of the third conventional structure shown in FIG. 41 as well, there aries a substantially similar problem.
Also, in case where the male spline portion 47 is formed into a shape having a torsion angle, since the mutually associated teeth of the male and female spline portions 47, 48 are pressed against each other only in the axial-direction two end portions thereof, there is a possibility that the contact surface pressure acting on the side surfaces of these teeth can increase up to an excessively high value. And, in case where the contact surface pressure becomes excessively high, there is a possibility that the teeth can be worn or plastically deformed in a relatively early stage. For this reason, the sufficient durability of the male and female spline portions 47, 48 cannot be secured, which makes it difficult to prevent occurrence of the teeth striking sounds for a long period of time.
A wheel-driving unit and a method for manufacturing the same according to the invention are invented in view of the above-mentioned circumstances of the prior art.